feat(chat): Improve markdown handling

[gadenbuie]

Problem

This PR attempts to solve the problem of wanting to use HTML intermixed in the markdown given to the streaming markdown component. This is complicated by the fact that it's common to use html-like tags to mark up parts of the prompt, so we might want to render <img> but have <example> escaped to appear in the view. Currently, (when writing up this PR at least) all HTML is escaped, so it wasn't possible to add an image using HTML.

Background

Initially, this PR started by allowing content_type on <shiny-user-chat> to customize the content type of the user message (previously hard-coded to `"semi-markdown").

I still think that's a reasonable thing to do, but I think we could simplify and have a single "markdown" mode. That said, I've kept markdown and semi-markdown for now, but I've refactored them a bit. semi-markdown now lets more HTML through while escaping any unknown tags to retain them in the markup.

We can consolidate into markdown, html and text message modes by reverting b90c211.

Example prompts

<img src="https://placehold.co/200x200"> ![](https://placehold.co/200x200) <example>thing</example>

Before	After

Note that if we made markdown and semi-markdown the same thing, the echoed output from this assistant would match the user message presentation.

You’re a financial analyst at **AcmeCorp**. Generate a Q2 financial report for our investors.

This is an 'unusual' character: "&".

Use this data for your report:<data>{{SPREADSHEET_DATA}}</data>

<instructions>
1. Include sections: Revenue Growth, Profit Margins, Cash Flow.
2. Highlight strengths and areas for improvement.
</instructions>

Make your tone concise and professional. Follow this structure:
<formatting_example>{{Q1_REPORT}}</formatting_example>

Before	After